Approximately 60% of all streams in the conterminous U.S. are ephemeral or intermittent. In the southwestern U.S., they make up more than 80% of all water courses. While dry during most of the year, ephemeral streams (also called arroyos) episodically convey runoff and sediment following intense summer storms. Arroyos provide a wide range of ecological functions such as habitat and hydrologic connectivity. Perhaps less notably, they also furnish ecosystem goods and services important for human well-being: groundwater recharge, water quality benefits, and recreation, to name just a few examples. Despite their vast spatial extent, dryland watersheds, and the arroyo systems they support have received less attention compared to their perennial counterparts. Under natural conditions, arroyos are in a state of dynamic equilibrium between sediment supply and transport. This equilibrium is disrupted by urbanization. In urban areas, impervious cover causes increased surface runoff. This, in turn, increases the magnitude and frequency of runoff events. At the same time, imperviousness prevents erosion, and runoff from municipal areas carries little sediment. Increased discharge and decreased sediment supply lead to channel degradation. In the past, runoff from urbanizing areas was often treated as a nuisance, and arroyos were seen as having one sole purpose: to get rid of stormwater as fast and efficiently as possible. This short-sighted view of ephemeral systems overlooks the multitude of ecosystem services they provide. Channel degradation has a profound negative impact on the welfare of dryland watersheds and the people that inhabit them. This study examines one arroyo in central New Mexico where urbanization-induced stressors have led to substantial degradation. Engineered elements designed to increase channel stability have had the opposite effect and exacerbate the problem. Important ecosystem services provided by the stream are identified, and possible restoration strategies are discussed. This example demonstrates that low impact development in a semi-arid setting requires rethinking of traditional engineering approaches.
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